Acid-base responsive switching between 3+1 and 2+2 platinum complexes

Article abstract of DOI:10.1039/c3cc46587j

We report that the acid-base induced changes to a cyclometallated platinum complex can be used to drive the exchange of accompanying ligands with different denticities.

Full text of DOI:10.1039/c3cc46587j

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Acid–base responsive switching between ‘‘3+1’’
and ‘‘2+2’’ platinum complexes†
Dhassida Sooksawat,a Sarah J. Pike,a Alexandra M. Z. Slawinb and Paul J. Lusby*a
Cite this: Chem. Commun., 2013,
49, 11077
Received 28th August 2013,
Accepted 15th October 2013
DOI: 10.1039/c3cc46587j
www.rsc.org/chemcomm
We report that the acid–base induced changes to a cyclometallated
platinum complex can be used to drive the exchange of accom-
panying ligands with different denticities.
Central to the development of synthetic molecular machines is the
discovery of new externally addressable, switchable molecular or
supramolecular elements.1 Bistable rotaxanes and catenanes which
utilise transition metal–ligand interactions are particularly attractive,2–5
not least because their inherent strength can be used for purposes such
as ratcheting.6 Another desirable feature of switchable systems is that
they should exhibit a large thermodynamic bias in both states. In this
regard, the benchmark for systems where the metal ion remains
integrated during operation4 is still Sauvage’s exploitation of the
preferred coordination differences of Cu(I) and Cu(II), which was first
Fig. 1 X-ray crystal structures of (a) [HL1Pt(dmbipy)]OTs and (b) [L1Pt(3,5-lut)].
Colour code: C(L1/HL1), blue; C(3,5-lut), red; C(dmbipy), green; N, light blue; Pt,
magenta. Counter anion and solvent molecules have been removed for clarity.
described nearly twenty years ago.2a Herein, we describe cyclometallated differently, fully consistent with the occupation of the trans-to-nitrogen
platinum motifs which are able to exchange monodentate and biden- and trans-to-phenylato coordination sites, the latter which experiences
tate ligands with excellent selectivity in response to acid–base stimuli. pronounced shielding from the non-coordinating phenyl group of HL1.
We recently reported that 2,6-diphenylpyridine (H2L1) can coor- The structure of [HL1Pt(dmbipy)]OTs, confirmed by X-ray crystallogra-
dinate to platinum(II) either as a doubly anionic tridentate or phy (Fig. 1a) using crystals grown from diisopropyl ether and chloro-
monoanionic bidentate ligand, and that this can be exploited to form, shows that while platinum adopts a near planar geometry (the Pt
assemble–disassemble metallosupramolecular architectures based on ion sits just 0.078 Å above the mean N3C coordination plane) there is
cis-coordinating multitopic pyridyl ligands.7 As well as accepting two significant distortion and twisting of the ligands to alleviate steric clash
monodentate ligands, we envisaged that a platinum complex where between the non-coordinating phenyl group and the adjacent dmbipy
HL1 coordinates in a Z2 C4N fashion would be able to accommodate a ligand such that the complex adopts an overall helical twist.8 In the
neutral bidentate ligand. Thus, when [L1Pt(DMSO)] was treated with a solid state, both P and M enantiomers are observed, although in
mixture of para-toluene sulfonic acid (TsOH) and 5,50-dimethyl-2,20- solution at room temperature the interconversion appears to be rapid.9
bipyridine (dmbipy) in CH2Cl2, a compound was isolated which
To explore whether other neutral monodentate ligands would be
spectroscopic data indicated was the ‘‘2+2’’ complex, [HL1Pt- similarly displaced, experiments starting with [L1Pt(py)], [L1Pt(3,5-lut)]
(dmbipy)]OTs. For instance, the 1H NMR spectrum (see ESI†) showed and [L1Pt(DMAP)] (py = pyridine; 3,5-lut = 3,5-lutidine; DMAP =
that the two pyridyl ‘‘halves’’ of the dmbipy ligand resonate quite 4-dimethylaminopyridine) have been undertaken (Scheme 1). In the
1
absence of TsOH, H NMR spectroscopy shows that, in all cases,
dmbipy does not displace the monodentate ligand and remains
uncoordinated (see the ESI,† Fig. S1–S3). For [L1Pt(py)], the addition
of a slight excess (1.3 eq.) of TsOH to the charge neutral complex and
dmbipy results in the rapid displacement of the monodentate ligand
and concomitant generation of [HL1Pt(dmbipy)]OTs (Scheme 1 and
Fig. S1, ESI†). When similar ligand exchange experiments were carried
out with [L1Pt(3,5-lut)] and [L1Pt(DMAP)], it was found that the
addition of over two equivalents of TsOH was required in order to
a EaStCHEM School of Chemistry, University of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh, EH9 3JJ, UK. E-mail: Paul.Lusby@ed.ac.uk;
Fax: +44-131-6506453; Tel: +44-131-6504832
b School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews, Fife,
UK KY16 9ST
† Electronic supplementary information (ESI) available: This includes all syn-
thetic procedures, compound characterisation, ligand exchange experiments and
crystallographic data. CCDC 956076–956078. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3cc46587j
c
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Scheme 1 Acid–base interconversion between ‘‘3+1’’ and ‘‘2+2’’ cyclometal-
lated Pt complexes. Conditions: a 1.3–2.7 eq. TsOH, CD2Cl2, 298 K, 5 min; b P1-tBu,
CD2Cl2, 298 K, 1.5–2 h.
generate the ‘‘2+2’’ complex (Fig. S2 and S3, ESI†). We attribute this
difference to two main factors – (i) the increased basicity of the
liberated 3,5-lut and DMAP heterocycles, which leads to partial
re-cyclometallation and (ii) the formation of more stable ‘‘2+1+1’’
intermediates (e.g. [HL1Pt(DMAP)OTs] and [HL1Pt(3,5-lut)OTs]), from
which the chelate-driven displacement of DMAP/3-5-lut and tosylate
ligands by dmbipy requires additional proton-assistance (for further
discussion of these processes, see the ESI†).
Scheme 2 Synthesis of, and acid–base responsive interconversion between,
‘‘3+1’’ and ‘‘2+2’’ macrocyclic cyclometallated Pt complexes. Conditions: a CH2Cl2,
313 K, 16 h, 37%; b CH2Cl2, 298 K, 1 h, 34%; c 1.7 eq. TsOH, CD2Cl2, 298 K, 5 min;
As the pyridyl-based ligands are not replaced in the absence of acid,
we predicted that simply adding base to a mixture of [HL1Pt(dmbipy)]-
OTs and monodentate moiety would reverse the coordinative bias. We
have previously used the phosphazene base, P1-tBu, to affect (re)cyclo-
metallation.7 Thus, when P1-tBu was added to the mixture of
[HL1Pt(dmbipy)]OTs and monodentate pyridyl ligand (py, 3,5-lut or
DMAP), the gradual disappearance of signals due to [HL1Pt(dmbipy)]-
OTs and re-appearance of [L1Pt(py)], [L1Pt(3,5-lut)] and [L1Pt(DMAP)]
followed 1.5–2 h at room temperature (Scheme 1 and Fig. S1–S3, ESI†).
To develop the system further, and with a stimuli-responsive
molecular shuttle in mind, the macrocyclic C4N4C Pt complex,
[L2Pt(DMSO)], has been prepared. The free macrocycle, H2L2, was
obtained in an excellent 80% from 2,6-di-(4-hydoxyphenyl)pyridine10
and a bisphenol A derived dialkylbromide,11 while insertion of
platinum also occurs in an highly respectable 80% yield (see ESI†).
From [L2Pt(DMSO)], [L2Pt(3,5-lut)] was prepared by exchange of the
coordinated DMSO for 3,5-lut (Scheme 2, step a). The structure of
[L2Pt(3,5-lut)] has been confirmed by X-ray crystallography using
single crystals grown from slow diffusion of diethyl ether into a
saturated dichloromethane solution (Fig. 2). In solution, the
1H NMR spectrum of [L2Pt(3,5-lut)] (Fig. 3a) exhibits coordinated
ortho lutidine signals (Ha) with characteristic 195Pt satellites, while
the para site (Hb) is significantly upfield shifted with respect to the
d
5 eq. P1-tBu, CD2Cl2, 298 K, 48 h.
Fig. 2 X-ray crystal structure of [L2Pt(3,5-lut)]. Colour code: C(L2), blue; C(3,5-lut),
red; N, light blue; O, orange; Pt, magenta. Solvent molecules have been removed
for clarity.
free heterocycle, most likely due to shielding by the bisphenol A unit. spectrum of [HL2Pt(L3)]OTs (Fig. 3e) is the sterically congested nature
The TsOH induced formation of
a ‘‘2+2’’ complex from of the complex, as the four triplets that arise from the HF, HG, Hg and
[L2Pt(DMSO)], this time using the unsymmetrical bidentate 2-(1-ethyl- Hh environments in the [L2Pt(3,5-lut)] and L3 precursors have been
phenyl-1H-1,2,3-triazol-4-yl)-5-bromopyridine (L3) was also carried out replaced by a complex diastereotopic pattern of signals.
1
(Scheme 2, step b). The H NMR spectrum of the product revealed
Finally, the ‘‘3+1’’ to ‘‘2+2’’ interconversion between [L2Pt(3,5-lut)]
several interesting features (Fig. 3e). Firstly, when it could be reasonably and [HL2Pt(L3)]OTs was examined using 1H NMR spectroscopy
expected that a mixture of both geometric isomers would result, only (Scheme 2 and Fig. 3b–d). When L3 was mixed with [L2Pt(3,5-lut)]
one – trans-[HL2Pt(L3)]OTs (see the ESI† for details of assignment using in CD2Cl2, only signals which are attributable to those two species
NOESY) – is produced. As the NMR of this product did not change over were apparent (Fig. 3b). Upon the addition of 1.7 eq. of TsOH, the
time, we would suggest that this product is both the kinetically and NMR spectrum changed to give a set of signals that were virtually
thermodynamically favoured isomer, as it has been previously observed indistinguishable from those of trans-[HL2Pt(L3)]OTs (Fig. 3c). As
that platinum isomers which feature cyclometallated ligands undergo with the acyclic system, this change occurred within the time taken
sluggish rearrangement.12 The second thing to note from the 1H NMR to record a subsequent spectrum. This rapid generation coupled to
c
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Fig. 3 Partial 1H NMR spectra (CD2Cl2, 500 MHz, 300 K) showing interconversion between ‘‘3+1’’ and ‘‘2+2’’ macrocyclic cyclometallated Pt complexes. (a) [L2Pt(3,5-lut)]; (b) a 1 : 1
mixture of [L2Pt(3,5-lut)] and L3; (c) a 1:1 mixture of [L2Pt(3,5-lut)] and L3, 5 minutes after the addition of TsOH (1.7 eq.); (d) 48 h after the addition of P1-tBu (5 eq.) to a solution of
[HL2Pt(L3)]OTs and 3,5-lut; (e) [HL2Pt(L3)]OTs. Macrocycle signals are shown in blue, 3,5-lut in red and L3 in green. The assignments correspond to the lettering shown in Scheme 2.
3 For interlocked systems which switch between a transition metal
bound state and one other state, see (a) P. Mobian, J.-M. Kern and
J.-P. Sauvage, Angew. Chem., Int. Ed., 2004, 43, 2392; (b) J.-P. Collin,
D. Jouvenot, M. Koizumi and J.-P. Sauvage, Eur. J. Inorg. Chem., 2005,
1850; (c) M. J. Barrell, D. A. Leigh, P. J. Lusby and A. M. Z. Slawin,
Angew. Chem., Int. Ed., 2008, 47, 8036; (d) Z. Xue and M. F. Mayer,
J. Am. Chem. Soc., 2010, 132, 3274.
the lack of observation of another isomer further supports that trans-
[HL2Pt(L3)]OTs is the kinetic (and thermodynamic) product, irre-
spective of the neutral monodentate ligand in the starting ‘‘3+1’’
complex. When five eq. of P1-tBu was added to the NMR sample
containing [HL2Pt(L3)]OTs and free lutidine, the signals due to the
‘‘2+2’’ complex and monodentate ligand started to disappear,
accompanied by the appearance of signals due to [L2Pt(3,5-lut)]
and free L3. After 48 h, only signals due to [L2Pt(3,5-lut)] and free L3
could be observed (Fig. 3d). The sluggishness of this reaction in
comparison to the same process for the acyclic complexes is almost
certainly due to an increased steric barrier. We are currently
investigating methods of lowering this by employing different
bases/anions and/or using light to accelerate ligand exchange.12a
In summary, a rare example of a metallosupramolecular switch
which can be alternated with excellent selectivity between different
states using acid–base inputs has been described. We envisage that
these types of externally addressable coordination complexes will
continue to play a significant role in the development of molecular
machines.
4 For interlocked systems which switch between different transition
´
metal-bound states through transmetallation, see (a) M. C. Jimenez,
C. Dietrich-Buchecker and J.-P. Sauvage, Angew. Chem., Int. Ed.,
¨
2000, 39, 3284; (b) J. D. Crowley, K. D. Hanni, D. A. Leigh and A. M. Z.
Slawin, J. Am. Chem. Soc., 2010, 132, 5309; (c) D. A. Leigh, P. J. Lusby,
A. M. Z. Slawin and D. B. Walker, Chem. Commun., 2012, 48, 5826.
5 For non-interlocked metallosupramolecular (mechanical) switches, see
(a) L. Fabbrizzi, F. Gatti, P. Pallavicini and E. Zambarbieri, Chem.–Eur. J.,
1999, 5, 682; (b) S. Akine, S. Hotate and T. Nabeshima, J. Am. Chem. Soc.,
2011, 133, 13868; (c) G. Tsekouras, O. Johansson and R. Lomoth, Chem.
Commun., 2009, 3425; (d) K. A. McNitt, K. Parimal, A. I. Share, A. C.
Fahrenbach, E. H. Witlicki, M. Pink, D. K. Bediako, C. L. Plaisier, N. Le,
L. P. Heeringa, D. A. V. Griend and A. H. Flood, J. Am. Chem. Soc., 2009,
131, 1305; (e) X. Su, T. F. Robbins and I. Aprahamian, Angew. Chem.,
Int. Ed., 2011, 50, 1841; ( f ) M. Schmittel, S. De and S. Pramanik,
Angew. Chem., Int. Ed., 2012, 51, 3832; (g) C. W. Machan, M. Adelhardt,
A. A. Sarjeant, C. L. Stern, J. Sutter, K. Meyer and C. A. Mirkin,
J. Am. Chem. Soc., 2012, 134, 16921.
6 (a) M. N. Chatterjee, E. R. Kay and D. A. Leigh, J. Am. Chem. Soc.,
2006, 128, 4058; (b) V. Serreli, C.-F. Lee, E. R. Kay and D. A. Leigh,
Nature, 2007, 445, 523; (c) A. Carlone, S. M. Goldup, N. Lebrasseur,
D. A. Leigh and A. Wilson, J. Am. Chem. Soc., 2012, 134, 8321.
7 P. J. Lusby, P. Mu¨ller, S. J. Pike and A. M. Z. Slawin, J. Am. Chem. Soc.,
2009, 131, 16398.
This work was supported by the EPSRC and The Royal
Society. P.J.L. is a Royal Society University Research Fellow.
Notes and references
1 E. R. Kay, D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed., 2007, 46, 72.
2 For interlocked systems which are able to switch between two
transition metal-bound states, see (a) A. Livoreil, C. O. Dietrich-
Buchecker and J.-P. Sauvage, J. Am. Chem. Soc., 1994, 116, 9399;
8 C. Deuschel-Cornioley, H. Stoeckli-Evans and A. von Zelewsky,
J. Chem. Soc., Chem. Commun., 1990, 121.
9 In situ generated [HL1Pt(dmbipy)]CSA (CSA = camphorsulfonate)
only shows a single set of 1H NMR resonances in CD2Cl2, a solvent
in which tight ion pairing could be expected. Similarly, the addition
of TRISPHAT to [HL1Pt(dmbipy)]OTs in CD2Cl2 does not show any
signal splitting. See the ESI† for details. The low energy interconver-
sion between diastereoisomers would suggest a mechanism which
doesn’t involve decoordination of dmbipy.
´
(b) D. J. Cardenas, A. Livoreil and J.-P. Sauvage, J. Am. Chem. Soc.,
1996, 118, 11980; (c) A. Livoreil, J.-P. Sauvage, N. Armaroli,
V. Balzani, L. Flamigni and B. Ventura, J. Am. Chem. Soc., 1997,
˜
119, 12114; (d) N. Armaroli, V. Balzani, J.-P. Collin, P. Gavina,
J.-P. Sauvage and B. Ventura, J. Am. Chem. Soc., 1999, 121, 4397;
´
¨
(e) J.-P. Collin, F. Durola, J. Lux and J.-P. Sauvage, Angew. Chem., Int. 10 V. Aucagne, J. Berna, J. D. Crowley, S. M. Goldup, K. D. Hanni,
Ed., 2009, 48, 8532; ( f ) J.-P. Collin, F. Durola, J. Lux and
J.-P. Sauvage, New J. Chem., 2010, 34, 34; (g) J. D. Crowley, D. A.
D. A. Leigh, P. J. Lusby, V. E. Ronaldson, A. M. Z. Slawin, A. Viterisi
and D. B. Walker, J. Am. Chem. Soc., 2007, 129, 11950.
Leigh, P. J. Lusby, R. T. McBurney, L.-E. Perret-Aebi, C. Petzold, A. M. 11 F. Durola, J.-P. Sauvage and O. S. Wenger, Helv. Chim. Acta, 2007, 90, 1439.
Z. Slawin and M. D. Symes, J. Am. Chem. Soc., 2007, 129, 15085; 12 (a) S. J. Pike and P. J. Lusby, Chem. Commun., 2010, 46, 8338;
(h) D. A. Leigh, P. J. Lusby, R. T. McBurney and M. D. Symes, Chem.
Commun., 2010, 46, 2382.
(b) O. Chepelin, J. Ujma, P. E. Barran and P. J. Lusby, Angew. Chem.,
Int. Ed., 2012, 51, 4194.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 11077--11079 11079